There is an undisputed need for rapid, accurate, reliable and inexpensive means for and method of testing and diagnosing malfunctioning in internal combustion engines of both the spark-ignition and the compression-ignition (diesel) types. The need can best be met by using electronic means, particularly by means including a minicomputer or microprocessor. One troublesome diagnostic task is determining whether the cause of irregular or inferior operation of an engine is due to causes affecting all cylinders in a random fashion, or to causes affecting only a particular one or ones of the cylinders. That is, parts of the ignition system and of the fuel-supplying system affect all cylinders randomly or equally, and other parts affect solely one cylinder. For example, a carburetor and an ignition timer each affect all cylinders, whereas a spark plug, a fuel injector, and a valve each affect only one cylinder.